The present invention generally relates to charge coupled devices (CCD's) and particularly those which are to be operated at high frequencies as shift registers.
Devices of the above type are characterized by a semiconducting substrate in which a relatively long and narrow channel is defined by a channel stopping doped region. Spanning, or bridging, the channel is a plurality of slender electrodes to which clock pulses are applied in sequence to cause an injected packet of charge to be stepped along the channel to successive storage sites created by the electrodes in the substrate within the channel. In injecting and extracting charge to and from such a CCD it has been customary to make the charge move directly toward or away from a given electrode through an inlet or outlet port in the side of the channel. This is particularly true in the case of a so called Racetrack CCD in which the main charge carrying channel has no end, so that read in and readout must, of necessity, occur through the side of the channel. Since the electrodes are much longer than they are wide, a charge which is being injected or extracted must traverse a much greater distance than one which is simply being stepped forward. As a result, the operating frequency of the CCD is greatly diminished by the need to accomodate the time required for charge read in and readout.
The reading of charge into or out of a CCD are but two applications of a broader concept of the present invention, which is the merging and splitting of charge. In the case of merging, charges progressing in an auxiliary channel are merged into charges flowing along a main channel of a CCD. The charge flowing in the auxiliary channel can be thought of as an input to the charges which flow in the main channel.
Similarly, in the case of charge splitting, charges which flow in a main channel are split off at a junction into charges which flow along the main channel and charges which are diverted to flow along the auxiliary channel. The "length of charge transfer time" problem caused by the electrode length/width ratio is present when merging or splitting is attempted.
It is, therefore, a general object of the present invention to provide charge merging and splitting junctions which do not slow down the operation of the CCD's with which they are used.
Another general object of the present invention is to increase the operating frequency of CCD shift registers. It is a more specific object of the invention to provide an input and an output for a CCD which does not significantly limit its operating speed.
As applied to the reading of information into a CCD, it is an object of the invention to provide an input station for a CCD through which a charge packet can be injected into the CCD in such a manner that, although the charge packet is injected into the CCD in a brief time period, it is progressively transferred into and stored by the CCD over several succeeding time periods.
Similarly, as applied to the reading of information from a CCD, it is a specific object of the invention to provide an output structure which is capable of receiving and accumulating over several successive time periods a charge packet which is to be read out during a subsequent single time period.
The charge-splitting junction of the present invention basically comprises a main charge-carrying channel defined in the surface of a semiconducting substrate by a pair of channel-stopping walls, one of which has a discontinuity along a predetermined portion thereof to permit charge to flow from the channel, and an auxiliary channel extending next to the main channel and communicating therewith through the discontinuity so as to receive charge flowing therethrough. The discontinuity through which the channels communicate may be either a single opening whose length is on the same order as is the width of the channels, or it may comprise a series of short spaced-apart openings whose width is a fraction of the widths of the channels.
An electrode array extends across both channels through the discontinuity so as to successively step along the main channel, and past the discontinuity, a first charge which is to be split, while concurrently stepping along the auxiliary channel a second charge which has been split from the first charge. As a result, the first charge will flow progressively along the electrode array from the main channel to the auxiliary channel as that charge is successively stepped along the main channel by the electrode array.
The charge-merging junction of the present invention operates on the same principle as the charge-splitting junction just described. It, too, is comprised of a main charge-carrying channel and an auxiliary channel intercommunicating through a discontinuity which is in a common wall between them and which may be either a single opening or a plurality of shorter openings. By the provision of an electrode array, extending across both of the channels through the discontinuity for stepping the charges along the main and auxiliary channels in tandem past the discontinuity, charge being stepped along the auxiliary channel may be caused progressively to flow along the electrode array through the discontinuity into the main channel as the charges in both channels are being successively stepped past the discontinuity.
As applied to a CCD having a semiconducting substrate, with means within the substrate defining a charge-carrying channel in which a series of storage sites are defined when an array of electrode structures distributed on the surface of the substrate are electrically energized with clocking signals applied to the electrode structures to step charge packets along the channel between successive ones of the storage sites, the objects of the present invention are attained by the provision of an improved read in station comprising means within the substrate defining a charge inlet chamber communicating with an enlarged section of the charge-carrying channel through an interconnecting port and by the combination with the charge inlet chamber of means whereby charge packets may be injected from the chamber through the port into the enlarged section of the channel in the direction of charge flow therein synchronously with the clocking signals which are being applied to the electrode structures.
In further keeping with the present invention, additional objects of the invention are attained in a CCD of the type just recited by the provision of an improved readout station featuring an auxiliary charge-carrying channel in the substrate next to the main charge-carrying channel, with the auxiliary channel communicating with the main channel throgh a series of connecting channels (hereinafter referred to in this specification as "transfer ports") in the substrate. The electrode structures which are distributed on the surface of the substrate, to define storage sites along the channel, terminate over the transfer ports, as do a second set of electrode structures, which extend across the auxiliary charge-carrying channel. Readout is effected by the provision of means for causing charge to flow from the main channel to the auxiliary channel through the transfer ports and by the further provision of means for stepping charge packets in tandem along both the main and auxiliary charge-carrying channels so that, during readout, a given charge packet is cumulatively transferred, at least in part, from the main channel to the auxiliary channel as the charge packet passes successive ones of the transfer ports.
Although most useful for reading (or splitting) charges out of a CCD, the multi-transfer port configuration may also be used for reading (or merging) charges into a CCD. The combination of both the improved readout and read in stations in a single CCD is to be preferred, since it results in the attainment of the largest number of the objectives of the present invention. However, either may be used without the other in specific circumstances.